Two-pump system for lift cylinder



y 13, 1969 A. KARAZIJA 3,443,380

TWO-PUMP SYSTEM FOR LIFT CYLINDER Filed Jan. 2, 1968 E; W (M .Q/P/PFA/WQQ United States Patent Office US. Cl. 60--52 4 Claims ABSTRACT OF THE DISCLOSURE A lift ram is supplied fluid by a primary circuit having a control valve and by a secondary circuit connected directly to the lift ram through one-way valve means. A load check valve is employed in the primary circuit upstream of the control valve and the secondary circuit is connected to the primary circuit upstream of the load check valve by a bypass passage with check valve permitting flow from the secondary passage to the relief valve for the primary pump located upstream of the load check valve in the primary circuit. The need of a separate relief valve for the fluid pump of the secondary circuit is thus obviated.

This invention relates to a hydraulic control system for a lift truck utilizing two pumps, the delivery of one of which is controlled by a control valve and the delivery of the other of Which is connected directly to the ram through a one-way valve.

Heretofore others have suggested dual pump delivery for a lift ram to achieve a high speed lift operation, however, such systems have required the expense of a relief valve for each of the pumps.

It is an object of this invention to provide a dual pump delivery system for a lift ram which requires only a single relief valve.

It is a further object of this invention to provide dual pump delivery for a lift ram wherein one pump delivers through a control valve with a load check valve upstream thereof, wherein the other pump delivers directly to the ram via a one-way valve and wherein an interconnecting passage with a check valve is provided between the output sides of the two pumps to eliminate the requirement for a second relief valve in the system.

These and others objects and advantages of this invention will be apparent to those familiar with the art when the following description is read in conjunction with the drawing schematically showing a preferred embodiment.

The present invention is schematically illustrated in a control system for a lift ram 11, the rod 12 of which carries a pulley 13 having a lift chain 14 reeved thereabout and connected at its opposite ends to the cylinder 16 of the ram 11 and to a carriage 17. The lower end of the ram 11 is pivotally connected to a lift truck chassis 18 and pivoting of the ram 11 about such pivot connection is controlled by a double acting hydraulic jack 19 having its opposite ends pivotally connected to the lift truck chassis 18 and the ram 11. It should be understood that the illustration is schematic and the mast construction of which the ram 11 is a part has been greatly simplified.

Hydraulic fluid is supplied to the lower end of the lift ram 11 by a primary pump 21 connected in driven relation to primary electric motor 22. A primary circuit 23, in which pump 21 is installed, includes a manually operated control valve 24 which may be operated by the vehicle driver through a control lever 26. One end of the primary control circuit is connected to an hydraulic reservoir 27 and the other end is connected through control valve 24 with the bottom end of the lift ram 11. A second control valve 31, operated by manual control lever 32,

3,443,380 Patented May 13, 1969 is provided for controlling operation of the tilt jack 19. When the control valves are in their illustrated neutral positions, the fluid delivery of primary pump 21 is routed from the primary circuit 23 to the reservoir by way of return to the reservoir conduit or passage 33. The primary circuit 23 is provided with a load check valve '34 which permits flow in the primary circuit to the lift ram but prevents flow from the lift ram back to the pump21.

Increased lift speed may be selected by the operator by supplying additional fluid to the ram 11 through a secondary circuit 36 which includes a secondary pump 37 driven by a secondary electric motor 38. One end of the secondary circuit is connected to reservoir 27 and the other end is connected to primary circuit 23 in fluid delivery relation to the lift ram 11.

A one-way valve in the for-m of a ball check valve 39 I is provided in the secondary circuit 36 permitting fluid flow from the secondary pump to the lift ram 11 but preventing a reverse flow so that the operator may employ only the primary pump 21 to raise the lift ram if he so desires. The control valve 24 will, of course, permit fine adjustment and control of the carriage raising speed.

The output of the primary pump 21 is relieved to its input side through a bypass passage 41 in which relief valve 42 is placed. The bypass passage 41 is connected to the primary circuit 23 in upstream relation to the load check valve 34 and is connected to reservoir 27 through return line 30. The secondary circuit 36 is connected to the primary circuit 23 at the output side of the pumps 37, 21 by an interconnecting passage 46 containing a check valve 47 which permits fluid flow from the secondary passage to the primary passage upstream of the check valves 39, 34. Check valve 47 prevents flow from the primary passage 23 to the secondary passage 36 This interconnecting passage 46 with check valve 47 eliminates the need for a second relief valve for the secondary pump 37 because it permits relief valve 42 to act as the relief valve for the secondary pump 37 as well as the primary pump 21.

Operation The illustrated system for controlling the lift ram 11 and tilt jack 19 has particular application in an electric powered lift truck. In such a truck, it has been found economical to use two hydraulic pumps since the combined capacity is only required when the operator desires extra high lift speed. The pumps are selectively operated only when it is desired to deliver pressure fluid to the lift ram or tilt jack. This is achieved by electric motor control means associated with the control valves.

When the operator desires to employ the output of primary pump 21 to extend the lift ram 11, the control valve 24 is adjusted downwardly by the control lever 26 from its illustrated hold position 51 to its raise position 52 in which fluid output is delivered through load check valve 34 to the bottom end of the lift ram 11. This is achieved by virtue of electric contacts 61, 62 having been bridged by contactor 63 carried by the control valve 24, thus energizing motor 22 by connecting it to the power source in the form of a storage battery 64.

If the operator desires a higher lift speed than that possible through the output delivery of primary pump 21, he will move control valve 24 downwardly a slight additional amount causing the contactor 6-6 on the valve 24 to bridge contacts 67, 68 thereby energizing secondary motor 38. The fluid delivery of pump 37 bypasses the control valve thereby eliminating the power loss that would result from forcing the additional fluid flow through the control valves 31, 24 which are tailored in size to efficiently handle no more than the output of primary pump 21. Fluid is delivered by the secondary pump 37 directly to the bottom of the lift ram by way of check 3 valve 39. When the rod 12 of the lift ram 11 reaches the end of its expansion stroke, the fluid delivery of primary pump 21 will be relieved to reservoir by way of relief valve 42 in bypass passage 41. The output of secondary pump 37 will also pass to and through the relief valve 42 by way of interconnecting passage 46 and check valve 47.

From the foregoing description it is apparent that the present invention provides a new and novel control system for a lift ram in which the requirement for a second relief valve for the secondary pump has been obviated. This is accomplished Without eliminating the desirable load check valve 34 from the primary circuit. It should be understood, of course, that when control valve 24 is moved upwardly to its lower position 53, the lower end of the lift ram will be connected to return to reservoir passage 45 and the contactor 66 will not bridge contacts 61, 6-2.

The embodiments of the invention for which an exclusive property or privilege is claimed are defined as follows:

1. In a hydraulic control system for a lift ram of the type having primary means for supplying hydraulic pressure fluid to said lift ram, including a primary pump, control valve, load check valve upstream of said control valve and relief valve means for relieving said system intermediate said primary pump and load check valve;

secondary means for selectively supplying hydraulic pressure fluid to said lift ram including a secondary pump connected to said lift ram through a check valve, and

means for relieving said secondary pump including passage means between the output of said secondary pump and said relief valve means and one-way valve means in said passage means preventing fluid flow therein in the direction toward said secondary pump and permitting fluid flow in the opposite direction when the delivery pressure of said secondary pump exceeds the delivery pressure of said primary pump.

2. The invention of claim 1 and further comprising an electric motor driving each of said pumps and control means for said motors permitting operation of the motor for said secondary pump only when said primary pump is delivering pressure fluid to said lift ram.

3. The invention of claim 1 wherein said secondary means includes a fiuid passage connecting said secondary pump to said lift ram independently of said control valve.

4. The invention of claim 3 including manual control means for controlling delivery of hydraulic pressure fluid to said lift ram by said secondary means.

References Cited UNITED STATES PATENTS 2,323,519 7/1943 Dean.

2,979,908 4/1961 Shook.

3,154,921 I l/1964 Junck et al.

3,171,247 3/1'96'5 McAlvay.

EDGAR W. GEOGHEGAN, Primary Examiner.

US. Cl. X.'R. 

